The processing and fabrication of conventional integrated circuits from semiconductor wafers is well known in the art. Typically, a semiconductor wafer which may comprise silicon is repeatedly patterned and processed to create a plurality of integrated circuits from a single wafer. Each of these integrated circuits is separated from one another by a scribe which encompasses each of the integrated circuits and which is formed during the patterning process. After completion of the circuit fabrication process, the wafer is sawed or cut along these scribe lines to produce individual circuit dies. The dies may later be encapsulated in plastic, ceramic or other packages which include circuit leads that are connected to the die by bond wires.
In conventional semiconductor processing, the fully processed semiconductor wafer is coated with a protective layer of oxide prior to the wafer being sawed into individual dies. Thus, each of the individual dies is protected from damage which may otherwise be caused by the debris particles generated during the saw process.
The scribe lines serve to seal each integrated circuit from the next so that no ion contamination is generated from the edges of the integrated circuit after a wafer saw break. The scribe lines are also used to provide alignment of reticles used during the repeated integrated circuit patterning process. The scribe lines can also serve to produce test structures within the scribe line, for instance, resistors or transistors, to provide a control monitor during processing. For instance, the resistance of a strip of the scribe can be tested to ascertain the resistance of an implanted material that may serve in the circuit section as the moat for transistor regions in the wafer. Conventionally, these scribe lines are only formed about the perimeter of the centrally located integrated circuits which are fabricated upon the wafer.
A recent innovation of Texas Instruments Incorporated of Dallas Texas, is the digital micromirror device or the deformable mirror device (collectively DMD). The DMD is an electro/mechanical/optical SLM suitable for use in displays, projectors and hard copy printers. The DMD is a monolithic single-chip integrated circuit SLM, comprised of a high density array of 16 micron square movable micromirrors on 17 micron centers. These mirrors are fabricated over address circuitry including an array of SRAM cells and address electrodes. Each mirror forms one pixel of the DMD array and is bistable, that is to say, stable in one of two positions, wherein a source of light directed upon the mirror array will be reflected in one of two directions. In one stable "on" mirror position, incident light to that mirror will be reflected to a projector lens and focused on a display screen or a photosensitive element of a printer. In the other "off" mirror position, light directed on the mirror will be deflected to a light absorber. Each mirror of the array is individually controlled to either direct incident light into the projector lens, or to the light absorber. The projector lens ultimately focuses and magnifies the modulated light from the pixel mirrors onto a display screen and produce an image in the case of a display. If each pixel mirror of the DMD array is in the "on" position, the displayed image will be an array of bright pixels.
For a more detailed discussion of the DMD device and uses, cross reference is made to U.S. Pat. No. 5,061,049 to Hornbeck, entitled "Spatial Light Modulator and Method"; U.S. Pat. No. 5,079,544 to DeMond, et al, entitled "Standard Independent Digitized Video System"; and U.S. Pat. No. 5,105,369 to Nelson, entitled "Printing System Exposure Module Alignment Method and Apparatus of Manufacture", each patent being assigned to the same assignee of the present invention and the teachings of each are incorporated herein by reference. Gray scale of the pixels forming the image is achieved by pulse-width modulation techniques of the mirrors, such as that described in U.S. Pat. No. 5,278,652, entitled "DMD Architecture and Timing for Use in a Pulse-Width Modulated Display System", assigned to the same assignee of the present invention, and the teachings of which are incorporated herein by reference.
The individual mirrors of the DMD mirror array are easily susceptible to damage from debris including particles generated during the wafer saw and break process. Because the DMD is a micromechanical device with movable pixel mirrors, the DMDs fabricated upon a wafer may not be conveniently covered with a protective oxide coating prior to a saw process as is conventional according to other semiconductor processing techniques. Moreover, due to the conductive address electrodes with are positioned below the conductive mirrors, a conductive particle entrapped between the mirror and address electrode could easily short the mirror to the address electrode. Thus, it is particularly important in the case of the DMD to avoid generating any particles during the wafer saw process which may degrade the performance of the mirrors.
As disclosed in commonly assigned U.S. Pat. No. 5,435,876 entitled Grid Array Masking Tape Process, one technique to protect the wafer during sawing is to utilize a grid array masking tape over the active surface of the processed wafer. The tape adheres to the wafer along a grid extending between the formed integrated circuits and prevents debris from damaging the active surface during the sawing process. The tape is removed after the saw process and then the photoresist under the mirror layer is undercut by a plasma etch process to form wells under the mirrors.
There is a need to provide an improved method for processing semiconductor wafers to produce micromechanical devices which reduces the likelihood that conductive particles are generated during the wafer saw process or remain entrapped under the mirrors after the plasma etch process of the photoresist spacer layer. This improved method should be applicable to fabricating integrated circuits having moving elements and which can not be readily provided with a conventional protective oxide coating.